Add support for `venv --prompt` (#1570)
## Summary
This PR adds the `--prompt` option to `venv` subcommand.
The default behavior for `uv venv` is to create a virtual environment in
the current directory with `.venv` name. This is different from `venv` /
`virtualenv` where a user always needs to provide the virtual
environment path. This allows us to define our own behavior in the
default scenario (`uv venv`). We've decided to use the current
directory's name in that case.
Workflows:
| Command | Virtual Environment Name | Prompt |
|--------|--------|--------|
| `uv venv` | `.venv` (default) | Current directory name |
| `uv venv project` | `project` | `project` |
| `uv venv --prompt .` | `.venv` | Current directory name |
| `uv venv --prompt foobar` | `.venv` | `foobar` |
| `uv venv project --prompt foobar` | `project` | `foobar` |
Fixes #1445
## Test Plan
This is my first Rust code and I don't know how to write tests yet.
I just checked the behavior manually:
```
$ cargo build
$ mkdir t
$ cd t
$ ../target/debug/uv venv -p 3.11
$ rg -w t .venv/bin/acti*
.venv/bin/activate.csh
13:setenv VIRTUAL_ENV '/Users/inada-n/work/uv/t/.venv'
20:if ('t' != "") then
21: setenv VIRTUAL_ENV_PROMPT 't'
23: setenv VIRTUAL_ENV_PROMPT "$VIRTUAL_ENV:t:q"
38: # in which case, $prompt is undefined and we wouldn't
.venv/bin/activate
48:VIRTUAL_ENV='/Users/inada-n/work/uv/t/.venv'
59: VIRTUAL_ENV_PROMPT="t"
.venv/bin/activate.fish
61:set -gx VIRTUAL_ENV '/Users/inada-n/work/uv/t/.venv'
73:if test -n 't'
74: set -gx VIRTUAL_ENV_PROMPT 't'
.venv/bin/activate.ps1
40:if ("t" -ne "") {
41: $env:VIRTUAL_ENV_PROMPT = "t"
.venv/bin/activate.nu
6:# but then simply `deactivate` won't work because it is just an alias to hide
35: let virtual_env = '/Users/inada-n/work/uv/t/.venv'
50: let virtual_env_prompt = (if ('t' | is-empty) {
53: 't'
```
---------
Co-authored-by: Dhruv Manilawala <dhruvmanila@gmail.com>
